If you dont mind the hetzer, then you will love the Stug III.
As for the IS, its a great tier 7 tank fully upgraded. I consider it only slightly inferior to the T-29 (fully upgraded) and much better than the Tiger. But until you get the big gun, it can be hard to hang with the larger tanks. If you are the top tier tank in a match, then you will probably be lead tank on a push - just be aware of the tactical situation and dont rush out on a flank alone. Heavy tanks are masters of the field, bar none, but alone they can be overwhelmed by smaller tanks. In larger battles you are more support for the even heavier tanks, and again, might have problems getting through their armor so make sure and take 10 to 20 rounds of HE just in case. Use HE when facing the front of anything larger than yourself, and AP for side/rear shots. Also, in larger battles, your better speed can sometimes enable you to chase down med tanks that break past the line. Again, dont hesitate to do this, as protecting your arty while eliminating enemy guns is important to getting more wins. In thinking of advanced crew training, I usually always go camo for all crew members as I often use my heavy tanks as a forward tank destroyer: I motor up to the lead and find some cover to hide in and watch the mini-map to see how things develop. If the enemy pushes hard on the flank i'm not on, I decide to either push foward and attack or fall back to defend. I would also recommend you buy the large caliber gun rammer as soon as you can - its critical for getting the most that you can out of those big guns.